This invention relates to a wiring board excellent in connection reliability and a process for producing the same.
A wiring board usually has a plurality of circuit conductors, and through-holes having metallized inner walls in portions necessary for connecting individual layers when the circuit conductors are formed on a plurality of layers.
Such a wiring board is mainly produced by a subtractive process or an additive process. The subtractive process comprises drilling or punching holes in an insulating substrate having conductors on the whole surfaces for connection on necessary portions, forming a metal on inner walls of the holes and the surface conductors by electroless plating or electrical plating, covering portions necessary for circuits with an etching resist, and removing exposed metal by etching to produce a wiring board. On the other hand, the additive process comprises drilling or punching holes on necessary portions for connection in an insulating plate, forming a plating resist so as to expose inner walls of the holes and portions necessary for forming circuits, and forming a metal on the inner walls of holes and the portions necessary for forming circuits.
Further, Japanese Patent Examined Publication (JP-B) No. 50-40466 discloses a process as shown in FIGS. 6(A) to 6(D) comprising forming conductor circuits 3 on surfaces of an insulating substrate 1 (FIG. 6(A)), covering the surfaces thereof with protective films 15 (FIG. 6(B)), drilling a hole 12 in the insulating substrate together with the circuit conductors (FIG. 6(C)), and forming a metal layer 14 on the inner wall of the hole 12 by electroless plating (FIG. 6(D)).
Still further, Japanese Patent Unexamined Publication (JP-A) No. 61-176193 discloses a process as shown in FIGS. 7(A) to 7(E) comprising removing a conductor in a size of through-hole diameter from portions 18 of a circuit conductor 3 on a front side necessary for connecting to a circuit of back side of a double-sided copper-clad laminate 17 (FIG. 7(A) and retaining a conductor on the back side corresponding to the connecting portions 18 to form patterns in desired form (FIG. 7(B)), forming a plating resist 25 on both sides (FIG. 7(C)), irradiating the connecting portions 18 on the front side with a laser light until the conductor of the circuit conductor 3 of the back side is exposed and removing the plating resist around the connecting portions 18 with the laser light (FIG. 7(D)), and forming an electroless plated film 24 on the inner walls of the holes and around the surface portions thereof together with the exposed portions of the circuit conductor 3 of the back side to produce a wiring board (FIG. 7(E)).
According to known subtractive processes and additive processes, since drilling or punching of through-holes is carried out at an initial stage of the production processes, fabricating conditions in the later steps for conductor circuits to be connected with the through-holes are remarkably limited in order to maintain the relation to the through-holes constant. For example, when a pattern called as a land which connects a through-hole and a circuit conductor is formed on a photomask which is a means for forming an etching resist, there must be taken into consideration expansion and contraction of the photomask and the insulating substrate used therein as well as an error of register at the time of working.
This becomes also a limiting factor in the formation of through-holes and a resist for electroless plating in the additive process.
Such a problem of hole positions in the subtractive processes and the additive processes does not take place in the process of the above-mentioned JP-B 50-40466. But, in recent wiring boards having a high wiring density, finer circuit conductors are also required. Thus, when such wiring boards are produced by the process of JP-B 50-40466, since the connecting area between a through-hole and a circuit conductor portion is small, break of circuits due to thermal stress readily takes place, which could not be expected from known wiring boards having low wiring density and results in producing a problem of lowering in connection reliability.
Further, when a laser light is used for forming holes as JP-A 61-176193, the problem of connection reliability and wiring density can be solved. But in the production of multi-layer wiring boards, there arise various problems in that it is necessary to laminate a plurality of double-sided wiring boards thus treated, to form holes necessary for connection and to make inner walls of through-holes electroconductive, an apparatus for forming holes at desired locations using a laser light is expensive, and the like.